By anaerobic curing adhesive compositions we means compositions which contain ethylenically unsaturated monomer which is stable for long periods when stored in presence of air or oxygen, but which polymerises to provide adhesive bonds rapidly in absence of air or oxygen particularly in the presence of certain metals. The invention relates especially to a method of locking nuts and bolts and the like by means of anaerobic adhesive compositions, and to selected adhesive compositions which may be used in such method.
It is known that certain ethylenically unsaturated compounds, including diacrylate esters of glycols (for example those glycols having one or more oxygen atoms in the chain) can be caused to polymerise by excluding them from contact with oxygen, and that polymerisation may be inhibited by maintaining the compounds in intimate contact with gaseous oxygen. It is commonly believed that the curing of anaerobic adhesives occurs by free radical polymerisation of ethylenically unsaturated monomers in the composition to provide polymer chains. It is known to promote polymerisation of these compounds by use of peroxides, and that the presence of hydroquinone enhances the shelf life of such products.
The balance between shelf life, polymerization speed and the required strength of adhesive bonds formed with anaerobic adhesive compositions has posed problems, and many proposals have been made to solve these problems. For example, in U.K. Pat. No. 1,090,753 it is proposed to employ selected peroxides in combination with an organic tertiary amine in an anaerobic composition including selected diacrylates, in order to provide a balance between shelf life and polymerisation speed of the composition. It has also been proposed to employ a hydroperoxide in conjunction with a tertiary amine. We have observed that anaerobic adhesive compositions containing a peroxide and an organic tertiary amine tend to have in most cases an acceptable shelf life but a less acceptable speed of polymerization to full bond strength.
It has also been proposed to include in anaerobic curing compositions an accelerator, e.g. an acid or a salt capable of dissociating in water to enhance the commencement of the polymerization. U.K. Pat. No. 1,161,661 is concerned with a nut locking adhesive composition based on selected acrylate monomers intended to provide satisfactory bonds not only on ferrous metal surfaces but also on surfaces normally less susceptible of adhesive bonding by anaerobic adhesive compositions, e.g. cadmium.
It has also been proposed to include in anaerobic curing compositions containing acrylate ester monomers a peroxy polymerization initiator, peroxide, an acidic substance and a substituted thiorea. U.S. Pat. No. 3,970,505 is concerned with an adhesive with ability to cure through relatively large gaps. It is indicated that the combination of acidic substance and substituted thiourea confers certain special and desirable properties on the adhesive. However, the substituted thiourea is so reactive with other components of the composition that it must be kept separate therefrom until use. In this case, not only are special substituted thioureas expensive to obtain, but also the adhesives must be stored and used as two part compositions.
We have found that anaerobic adhesives which include the more commonly available acids tend to be unsatisfactory for use on ferrous metals, because there is a tendency for the metal subsequently to become corroded as a result of corrosion-propagating action of residual acid remaining in the bond line after completion of the adhesive bond. Furthermore, we have found that a rapid curing of these adhesive compositions is more likely to be achieved by use of stronger acids, which in turn one would expect to lead to more severe corrosion problems. Naturally, corrosion is an undesirable result in view of the cost of replacing affected parts, and adhesives which actively encourage corrosion are regarded as undesirable.